Method and apparatus for obtaining ultramicroscopic images in an ion microscope



July 16, 1957 Filed Jan. 26, 1956 METHOD AND APPARATUS FOR OBTAININGULTRAMICRO SCOPIC IMAGES IN AN ION MICROSCOPE G. WEISSENBERG 2Sheets-Sheet 1 July 16, 1957 G. WEISSENBERG METHOD AND APPARATUS FOROBTAINING ULTRAMICROSCOPIC Filed Jan. 26, 1956 1 1 2| u) w K m y I m m fIMAGES IN AN ION MICROSCOPE 2 Sh e et -Sheet 2 IN V EN TOR.

METHOD AND APPARATUSFOR OBTAINING ULTRAli/HCRQSCQPIC IMAGES AN IONMICROSCQPE Gustav Weissenberg, Wetzlar (Lahn), Germany, assignor toErnst Leitz G. in. b. 1 1., Wetzlar, Germany Application .lanuary 26,1956, Serial No. 561,587 Claims priority, application Germany February1, 1955 17 Claims. (oi. zen-49.5

The present invention relates to methods and apparatus forultramicroscopy, and more particularly to methods and apparatus of thischaracter in which ions are used for the formation of a magnified image.

It is well known in the art that the resolving power of ion microscopesis theoretically greater than the resolving power of electronmicroscopes; This is because of the fact that the mass of an ion isgreater than that of an electron in a ratio of at least three powers often. However, the practical utilization of this higher resolving powerwhen using ions has been rendered difficult by this same factor, i. e.the relatively large mass of the ions and also by the unavailability ofany luminous screen or photographic material which is capable ofwithstanding the ion bombardment involved in the formation of themagnified ultramicroscopic image. At the same time, in known ionmicroscopes, of course, the full useful magnification corresponding tothe resolving power must be obtained.

Difficulties of this kind are avoided by the method and apparatusaccording to the invention the method being characterized by two steps.In the first step, a magnified image of the object is formed by means ofions on a mirror-finished metal plate Which is preferably cooled to anextremely low temperature for instance by liquid air. When theimage-forming ions impinge on the metal plate they change the chargesthereof so as to form atoms. T he distribution pattern of the atomsformed on the metal plate forms an invisible image of the object, aso-called latent image, on the metal plate. In the second step the metalplate is arranged as a cathode in an electron microscope and excited toproduce electron emission so that an image of this cathode is formed bymeans of an electron optical system on a luminous screen or aphotographic plate in a manner known per se in the art. The ionmicroscope may be arranged either as a transmissiontype microscope or asa reflection microscope. In a particularly advantageous design, the ionmicroscope and 'the electron microscope are set up in such a manner thatthe polished metal plate is used both for forming an ion image and as acathode without removing it from the vacuum. This may be accomplished,for example, by mounting the metal plate rotatably between an ionmicroscope and an electron microscope with their optical axes inalignment so that the polished surface is first presented at the imageplane of the ion microscope for formation of the latent image. The plateis then rotated through 180 which brings the polished surface intoposition at the object plane of the electron microscope, whereafter itis excited to cause electron emission and the magnified electron imagemay be observed directly on a fluorescent screen or recorded on aphotographically sensitized surface in conventional manner.

According to another embodiment of the invention, the ion microscope andthe electron microscope may be arranged with their optical axesangularly inclined with re- ,spect to each other and with the polishedsurface of the States Patent ice . r 2 metal plate obliquely inclinedwith respect to the optical axis of the ion microscope.

Accordingly, it is an object of the invention to provide anionmicroscope of high resolving power in which an invisible ion image isformed on a retentive surface, the invisible image being reproduced forvisual observation by electron emission, using either a fllorescentscreen or a photographic film, as desired. V A further object of theinvention is the provision of cathode means in the form of a mirrorfinished metal plate upon which a latent image is formed by ionbombardment, the latent image being transformed into an electron imageby activation of the plate to produce electron emission from itspolished surface.

Still another object of the invention is the provision of a cathodicplate of this character which is cooled during ion bombardment toprevent atom migration and which may be subsequently heated or otherwiseactivated to obtain electron emission therefrom.

Briefly, the invention comprises a source of ions, a lens system fordirecting the ions on or through the object to obtain an image, afurther lens system for focusing a magnified ion image on a polishedmetal plate to obtain an invisible latent image, and an electronmicroscope in which the polished surface of the metal plate is arrangedas a cathode at'its object plane, together with means for activating thepolished surface -to obtain electron emission therefrom.

Other objects, featuresand advantages of the invention will becomeapparent upon reading the following specification together with theaccompanying drawing forming a part hereof.

Referring to the drawing:

Figure 1 is a diagrammatic sectional view of an embodiment of theinvention, showing a reflecting ion microscope and an electronmicroscope having aligned optical axes and a rotatable metal plate orcathode; Figure 2 is a diagrammatic sectional view of a modifiedembodiment of the invention, showing an irradiating ion microscope andan electron microscope arranged at an angle to each other with the metalplate arranged at an inclination with respect to the optical axis of theion microscope; and

Figure 3 is a diagrammatic sectional view of a further embodiment of theinvention, showing a reflecting ion microscope and an electronmicroscope having an axis inclined with respect to the axis of the ionbeam from which an electron image is to be formed, the metal plate beingactivated to produce electron emission by ion bombardment.

Referring to Fig. 1, an evacuated vessel 1 is provided with a lateralextension 2. Disposed in the upper end of vessel 1 is an ion source 3and a condenser lens 4. The object 5 is disposed near the outer end ofthe lateral extension 2. Ions emanating from source 3, after passagethrough the condenser lens 4, are deflected by deflecting fieldstructure 6 through an angle of slightly less than toward the object 5,a diverging or dispersion lens 7 being provided near the object 5. Afterreflection from the object 5, the ions pass again through the diverginglens 7 and deflecting field 6 and thence through one or more projectionlenses 8 arranged in front of the polished surface of a metal ionreceiving plate 9. The metal plate 9 may be rotated at will by means ofa knurled knob 10 disposed exteriorly of the envelope 1, the plate 9being rotatably supported on a pivot pin 11 which extends through thewall of the evacuated envelope 1. The pivot pin 11 is formed of material having a high coefficient of thermal conductivity and is inthermal communication with suitable cooling means such as a supply ofliquid air (not shown). An electric heater 12 is disposed in a cavity inthe rear of the polished surface of plate 9 and is supplied with currentwhen it is desired to activate the polished surface as described ingreater detail below.

When the plate 9 is rotated through 180 on pivot pin 11, its polishedsurface is then located at the object plane of an electron microscopecomprising a luminous or flucrescent viewing screen 13 at the lower endof vessel 1, an objective lens 14, preferably of the immersion type, inproximity to the plate 9, and a projection lens 15 intermediate theimmersion lens 14 and viewing screen 13. It will be observed that theoptical axis through members 34-8 of the ion microscope portion of theapparatus is in alignment with the optical axis through members 141513of the electron microscope portion.

The operation of the apparatus is as follows:

The ions emanating from the ion source 3 and subjected topost-acceleration in conventional manner are focussed by the condenserlens 4 into a beam of parallel rays which is laterally deflected by thedeflecting field 6 towards the object 5. These ions are reflected fromthe object 5 in accordance with the varying ion-reflective properties ofdifferent portions of its surface thereby modifying the reflected beamof ions in conformity with pictorial details of configuration of theobject 5. The object 5 is at approximately the same potential as the ionsource 3. Alternatively, there may be provided an electricallyconductive surface at approximately the same potential as the ion source3 and from which the ions are reflected. In such case, the electricallyconductive surface will be superposed directly on the object 5 orlocated directly in front of the object and will present the samepictorial characteristics from an ion reflection standpoint as thosecharacteristics of the object which are to be observed by means of theapparatus.

The reflected ions are accelerated toward the deflecting field 6 by thediverging lens 7. These reflected ions are then deflected in thedeflecting field 6, this time to- Ward the plate 9. An ion-image ofobject 5 or its associated conductive surface is produced on thepolished surface of the plate 9 by the projection lens system 8. Theplate 9 is maintained at a negative potential With respect to the ionsource 3 and the object 5, it being assumed that the source 3 producespositively charged ions such as Cs ions, for example. These ions areattracted to the negatively charged surface of the plate 9 whereuponthey are transformed into a deposit of atoms, the density ofdistribution of this atomic deposit forming an invisible latent image ofthe object 5. In order to prevent migration of the atoms on the surfaceof plate 9, the plate is cooled to a low temperature as by liquid air.This cooling thus prevents loss of detail and preserves definition inthe latent image.

After formation of the latent atomic image on plate 9, it is rotatedthrough 180 by means of knob 10 so that its polished surface faces theobjective lens 14 and is directed toward viewing screen 13, the polishedsurface being normal to the optical axis of the electron microscopeportion of the apparatus. The plate 9 is then heated to electronemitting temperature by energization of heater 12. The polished surfaceof the plate 9 then operates as a cathode and the electron-opticalsystem comprising electron lenses 14-and 15 produces a visible image onthe viewing screen 13 in conventional manner. The plate 5? may be formedof metal such as tungsten, nickel or other electrode material which isadapted to be subjected to ion bombardment and activated to produceelectron emission.

By using ions, such as those of Cs having a low work factor, the latentimage on plate 9 is readily converted into a visible image on the screen13. It is to be understood, of course, that viewing screen 9 may bereplaced by a suitable photographically sensitized surface if apermanently recorded image is desired. By suitably dimensioning therelative magnifications within the ion mi croscope and electronmicroscope portions of the apparatus, the higher resolving power of theions may be fully utilized while at the same time, the disadvantagesinherent in ion microscopes of conventional construction are avoided.

In the modified embodiment of the invention shown in Fig. 2, the opticalaxes of the ion microscope and electron microscope portions of theapparatus are angularly inclined with respect to each other instead ofbeing in alignment as in the case of Fig. 1. An evacuated envelope 22contains an ion source 23. Ions from source 23 are focussed by acondenser lens 24 on an ion-translucent object 25. After passage throughthe translucent object 25, the ions are acted upon by a projection lenssystem 27, 29 so that a magnified image of object 25 is formed on thepolished surface of a metal plate 30. Metal plate 39 is maintained at asuitably low temperature by means of a coolant, such as liquid air,supplied through a duct 32 which communicates with the interior of plate30. The latent atomh: image formed on plate 30 is reproduced on aviewing screen 35 by means of an immersion type objective lens 36 and aprojection lens 37. The polished surface of plate 3'9 is inclinedobliquely with respect to the optical axis 38 through members 24-2729 ofthe ion microscope portion of the apparatus. The electron microscopeportion of the apparatus is disposed in an extension 39 of evacuatedenvelope 22 and its optical axis is normal to the polished surface ofplate 30.

The invisible latent image formed by ion bombardment of plate 30 isactivated by an electron source 40 and a condenser lens 41 which directan energy-rich stream of electrons against the polished surface of plate30. This electron stream is of suflicient intensity to excite the atomsformed thereon by the ion-image projection to a condition of secondaryelectron emission. The ion-target surface of plate 30 is thus caused toact as an image producing cathode in the electron microscope portion 35,36, 37 of the apparatus without manipulation of the metal plate as inthe case of the apparatus of Fig. l. The secondary emission electronsare then accelerated by the immersion type objective lens 36 andprojection lens 37 so that they strike the viewing screen 35 producingthe desired enlarged image of object 25. The parallax effect caused byobliquity of the polished surface of plate 30 with respect to the ionstream along optical axis 38 may be readily corrected by means ofcompensatory astigmatism included in lens system 36-37.

Figure 3 shows a further embodiment of the invention as applied to amicroscope of the reflecting type. In this instance, the paths of travelof the incident and reflected ion beams define an angle with each otherwhich is less than The optical axis of the electron-optical portion ofthe microscope is obliquely inclined with respect to the image-formingion beam which impinges on the metal plate. As in the case of Fig. 2,the ion beam which forms the latent image strikes the polished surfaceof the metal plate obliquely and the optical axis of the electronmicroscope portion of the apparatus is normal thereto.

A stream of ions emitted from a source 51 is directed through amonochromatic filter 52 which passes only ions having energy levelswhich lie within a narrow range. The selected ions then pass through acondensing lens 53 and a deflecting field structure 54 to the object 55,through a dispersion lens 56 being located in proximity to the object55.

The ions reflected from the object 55 pass back through the dispersionlens 56 and again through the deflecting field structure 54 to a seriesof magnifying projection lenses 57 which form an ion-image on thepolished surface of a low temperature metal plate 58. The electronmicroscope portion of the apparatus comprises an objective lens 6! aprojection lens 61 and a viewing screen 62. The ions which are trappedand converted to atoms on the polished surface of plate 58 are activatedto a state of secondary emission by bombardment with ions from a furtherion source 63, these activating ions being focussed on the surface ofplate 58 by a condenser lens 64. All of the apparatus is disposed withina common evacuated vessel.

When ions having an extremely small work function such as Cs, Rb, etc.are used, the electrode emission may be excited instead of by thermalemission or excitation to secondary emission by electron or ionbombardment, by irradiation with light and particularly ultravioletlight. The metal plate is illuminated during the image formation, byions or with the filtered light from a mercury lamp, the ion exposure ofthe object being interrupted when there appears on the luminous screenan electron image of suiiicient brightness.

If relatively heavy ions having a high work function are used, abombardment with electrons or ions is preferred.

The ion-optical and electron-optical lenses and other elements used maybe electrostatic and/or electromagnetic lenses or prisms. Furthermore,the individual parts shown in the embodiments may be combined with eachother to form compound elements, if desired.

In order to erase or cancel the latent image on the metal plate, it isonly necessary to heat the plate to an intense heat by means of theheater 12 until the imageforming atoms have been evaporated from itspolished surface. The latent atomic image may also be removed bysubjecting the polished surface of the plate temporarily to an unusuallyintense bombardment with atoms and/ or electrons.

While there have been disclosed what are believed to be the bestembodiments of the invention, it will be apparent to those skilled inthe art that many changes and modifications may be made therein withoutdeparting from the spirit and scope of the invention as defined in theappended claims.

What is claimed is:

1. The method of producing a visible ultramicroscopic image by ionbombardment of an object to be viewed, said method comprising the stepsof: bombarding said object with ions traveling in optical paths toobtain an ion-image of said object; enlarging and focusing said imageupon the polished surface of a metallic plate; exciting said surface toderive electron emission therefrom depicting said image; and directingsaid emitted electrons to utilization means for deriving an enlargedvisible image of said object therefrom.

2. The method according to claim 1, wherein said exciting step isperformed by heating said surface to a temperature at which electronemission occurs.

3. The method according to claim 1, wherein said exciting step isperformed by bombardment of said surface with charged particles toobtain secondary electron emission therefrom.

4. The method according to claim 3, wherein said surface is bombardedwith electrons.

5. The method according to claim 3, wherein said surface is bombardedwith ions.

6. The method according to claim 1, wherein said object is bombardedwith caesium ions and said polished surface is maintained at a negativepotential for converting said ions to caesium atoms.

7. The method according to claim 1, wherein said ions are positivelycharged metallic ions, said method comprising the further steps ofmaintaining said surface at a negative potential for converting saidions to atoms, and

cooling said surface to prevent migration of said atoms.

8. Microscopic apparatus of the class described, comprising incombination: an evacuated envelope; a source of ions disposed in saidenvelope; a metallic plate disposed in said envelope, said plate havinga polished surface; ion-optical means arranged to direct ions from saidsource to said surface, said ion-optical means comprising means fordirecting said ions to and from an object to be viewed and forming anenlarged latent image of said object on said surface; means foractivating said surface to cause electron emission therefrom forelectronically reproducing said latent image; and electron-opticalmicroscope means comprising objective lens means focussed on saidsurface for enlarging and reproducing said latent image duringactivation of said surface by said activating means.

9. Apparatus according to claim 8, wherein said source of ions producespositively charged metallic ions and in which said plate is maintainedat a negative potential for converting said ions to atoms, saidapparatus further comprising cooling means in thermal communication withsaid plate for reducing the temperature of said surface to prevent themigration of said atoms.

10. Apparatus according to claim 9, wherein said source of ions producescaesium ions.

11. Apparatus according to claim 8, wherein said activating meanscomprises heating means in thermal communication with said plate forheating said surface to a temperature at which electron emission occurs.

12. Apparatus according to claim 9, wherein said activating meanscomprises a source of electrons and electronoptical means for bombardingsaid surface with electrons from said source, the intensity of saidbombardment being sufiicient to cause secondary electron emission fromsaid surface.

13. Apparatus according to claim 8, wherein said actie vating meanscomprises a further source of ions and further ion-optical means forbombarding said surface with ions from said further source, theintensity of said bombardment being suflicient to cause secondaryelectron emission from said surface.

14. Apparatus according to claim 8, wherein the optical axes of saidion-optical and said objective lens means are aligned in proximity toand at opposite sides of said plate, said apparatus further comprisingrevoluble mounting means supporting said plate and permitting rotationof said plate through an angle of for selectively presenting saidsurface either to said ion-optical system or to said objective lensmeans.

15. Apparatus according to claim 8, wherein said polished surface isnormal to the optical axis of said objective lens means, the opticalaxis of said ion-optical system being angularly inclined with respect tosaid optical axis of said objective lens means for directing said ionsobliquely against said surface.

16. Apparatus according to claim 15 further comprising astigmaticcorrection means included in said electronoptical microscope means forcorrecting for parallax caused by said oblique direction of said ionsagainst said surface.

17. Apparatus according to claim 15, wherein said activating meanscomprises a source of charged particles and means for directing saidparticles obliquely against said surface to produce secondary electronemission therefrom.

No references cited.

